The following studies will be conducted to elucidate the chemical mechanism of action of purine nucleoside phosphorylase: 1. Mechanistic studies to distinguish between A1 and A2 mechanisms employing 31PNMR; 18O/16O and 14C/12C kinetic isotope effects, isotope trapping experiments. 2. Synthesis, testing of affinity and photaffinity labels to elucidate the nature of the active site residues. 3. NMR studies on enzyme bound substrates and inhibitors to determine the presence of substrate distortion; 19F, 13C labelled external probes synthesized incorporated into affinity labels to test the active site environment.